Detection of gaseous chemical compounds is useful for a wide array of applications, including (without limitation) quality control in food processing; detection and management of fuel supplies and exhausts; and detection of drugs, explosives and dangerous or illegal substances. By way of example, detecting compounds in fuel vapors enables determination and verification of fuel ratings (e.g., octane ratings and ocetane numbers). As another example, volatile organic compound detection facilitates pollution control and monitoring environmental safety. Additionally, detecting odors from foods and beverages, such as alcoholic beverages, enables product identification and quality control in production. Furthermore, rapid detection of trace signatures from explosives improves security at airports and for border control and enhances quality control in production.
Transportation facilities have a variety of vehicles, run by a variety of petroleum fuels and emitting varying amounts of pollutants. Gasoline consists of a complex mixture of hydrocarbons, most of which are alkanes with 4-10 carbon atoms per molecule. The octane number of gasoline is a measure of its resistance to knock (i.e., detonation of unburned fuel/air mixture beyond the boundary of a flame front when the mixture is subjected to a combination of heat and pressure). The octane rating is measured by the value of research octane number (RON) and motor octane number (MON). The octane value shown on gas pumps in Europe and Australia is RON, while the octane value shown on the gas stations in the Canada and United States and some other counties is the average of the RON and MON and known as (R+M)/2. The difference between RON and MON is 8 to 10 points. Therefore, the “regular” 87 octane fuel in the United States and Canada would be about 91 to 92 octane in Europe and Australia.
The quality control of fuels plays a major role in reducing pollution from vehicles. Different types of gasoline are manufactured, distributed to gas pumps and sold to users under controlled government regulations and guidelines. To increase the octane rating, gasoline are mixed with several types of additives such as (i) oxygenates such as alcohol (ethanol, methanol, isopropyl alcohol and t-butanol) and ethers (Methyl Tertiary Butyl Ether (MTBE), tertiary amyl methyl ether (TAME), ethyl tertiary butyl ether (ETBE), etc. (ii) antioxidants and stabilizers (ethylene diamine and phenylene diamine), (iii) antiknock agents (tetra-ethyl lead, Methylcyclopentadienyl Manganese Tricarbonly (MMT), ferrocenetoluene and isooctane), (iv) lead scavengers (v) fuel dyes (such as solvent Red 24, Red 26, Yellow 124 and Blue 35) and (vi) hybrid compounds (such as detergent agent, combustion catalyst and corrosion inhibitor) in different countries. Such additives are controlled under the regulation of each country's environment protection agency.
In the United States, for example, fuel station pumps are inspected periodically (e.g., twice per year). Inspectors check supply meters, collect fuel samples and the samples to accredited laboratories for testing of octane and adulteration. Various types of lower priced chemicals such as kerosene, industrial solvents, ether, naphtha and other chemicals may be widely used for adulteration of gasoline. In addition, unbalanced and prohibited additives may be added to increase the octane grade. Such adulterated gasoline may compromise performance, increase emission of pollutants and deposits of carbon in the vehicle engine. Gasoline quality is checked in accredited laboratories by standard reference methods, which are time consuming and expensive. In addition, analytical instruments, calibration mixtures, consumable spares and trained personnel are required to conduct the test. A portable, low cost and easy to operate sensors system can be very useful to government authorities for rapidly and efficiently test the quality of gasoline and diesel. It potentially could be installed in the automobile vehicles so that drivers can have quick information about the quality of purchased gasoline.
Governmental agencies also regulate many volatile organic compounds (VOCs), particularly those that are hazardous, such as benzene, toluene, and xylene. Such compounds are extremely harmful if inhaled over their threshold limit value (TLV). TLV, a recommended guideline for industrial workplace exposure developed by the American Conference of Governmental Industrial Hygienists (ACGIH), is based upon time weighted average (TWA) exposure during an 8 hour/day and 40 hour/week work schedule. TLV-TWA for benzene is 0.5 ppm, toluene is 50 ppm, and xylene is 100 ppm.
Analytical instruments such as infrared spectroscopy and gas chromatography offer high sensitivity, but they cannot practicably be used for real-time measurement of VOCs in the field. They require large sample collection time and analysis time. In addition, they require reagents and skilled operators.
Human sensory panels are used for testing quality, color, taste and odor of foods and beverages. Human smell also has poor sensing reproducibility because of possible infection, fatigue, time of day and prior odors analyzed. Electronic noses consist of arrays of odor sensors which can be applied to monitoring the ripening of wine and cheese; quality assurance of raw foods and food products; cooking processes, fermentation processes; industrial processes such as flavoring and blending; benchmarking; packaging interaction effects; freshness and aging control. While known sensing devices have refined the olfactometry process, they have shortcomings. For example, they cannot continuously work for long time periods or operate remotely.
Alcohol, water and aroma are three major components of alcoholic beverages. The discrimination of alcoholic beverages using electronic noses has been reported based on alcohol content and not true differences in the aroma profiles. Several isolation approaches such as distillation, adsorption method, liquid-liquid extraction, desalcoholization and dehydration using gas chromatography procedures were applied to remove alcohol and water content from the beverages and to isolate aroma for detection using the electronic nose. All isolation approaches required setting of equipment, relatively lengthy analysis time and skilled operation.
What is needed is a cost-effective, easy to use, portable sensor system that operates without need of an external or integrated heater and also reliably detects a wide variety of odors and vapors with exceptional sensitivity, selectivity and response time. The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.